Electronically commutated motor

ABSTRACT

An electronically commutated motor and control means therefor. The electronically commutated motor has no dead zone and no overlap despite an imperfect position detecting signal. The transition of the commutation is carried out in a differential manner. Any fluctuation in the operating temperature and/or deviation of the supply voltage does not disturb the commutation. The generating torque of the motor is easily controlled by a small power without using any additional power control means. A constant torque motor and a constant speed motor are illustrated.

Q United States Patent [151 3,662,238 Kobayashi et al. 1 May 9, 1972[54] ELECTRONICALLY COMMUTATED 3,452,263 6/1969 Newell ..318/138 MOTOR3,483,456 12/1969 Brunner et a1. 318/254 X 3,274,471 9 1966 M l l ..3181 [72] Inventors: Kazutsugu Kobayashi; Hisayuki Matsu- 0 2a a 38 t Y hil H f OS k 3,448,359 6/1969 Engel ...3l8/138 5111;; 0s a I a o a3,486,099 12/1969 Brunner et al ..318/254 [73] Assignee: MatsushitaElectric Industrial Co., Ltd. Primary1l\'aminerG. R. Simmons Filed: May20 1969 Anorrxey-Wenderoth, Lmd & Ponack [21] Appl. No.: 826,191 [57]ABSTRACT,

An electronically commututed motor and control means [30] ForeignApplication Priority Data h f y 21, Japan -43/35220 The electronicallycommutated motor has no dead zone and May 22, 1968 Japan ..43/35252 nooverlap despite an imperfect position detecting signm' The transition ofthe commutation is carried out in a differential [52] U.S. Cl ..318/254,318/439 manner Any fluctuation in the operating temperature and/or [51llll. Cl. ..H02k deviation of the pp y voltage does not disturb thecommuta [58] Field ofSearch ..313/133, 254,439 on The generating torqueof the motor is easily controlled by a small power without using anyadditional power control [56] References cued means. A constant torquemotor and a constant speed motor UNITED STATES PATENTS are Illustrated-3,517,289 6/1970 Brunner et al ..318/138 5 Claims, 16 Drawing FiguresPATENTEDMM 9 m2 3,662,238

, SHEET 1 OF 4 INVENTORS KAZUTSUGU KOBAYASHI HISAYUKI MATSUMOTO YOSHIAKIIGARASHI BY WMQZMZ,

ATTORNEYS PATENTEBMAY 9 I972 3 662 238 SHEET 3 OF 4 FIGGQ FIG. 3b

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FIGBe FIGBF FlG.3h

INVENTORS KAZUTSUGU KOBAYASHI HISAYUKI MATSUMOTO YOSHIAKI IGARASHI 1ELECTRONICALLY COMMUTATED MOTOR This invention relates to improvedelectric motors, particularly to electronically commutated motors, andmore particularly to those of the position detecting type.

A motor which is compact, selfstarting, which has a v preselecteddirection of rotation, which has a smooth torque, and which is capableof operating on DC, is useful in electronic equipment for drivingvarious movable components. Portable tape recorders, for example,-require such motors which in addition have a substantially constantpredetermined speed (as capstan motors) or a substantially constantpredetermined torque (as reel motors). Up to this time many methods havebeen proposed relating to the commutation of electric current flowingthrough the armature winding, which methods ulitize electronic devicessuch as photosensitive elements cooperating with a light source and arotating slit, magnetic sensitive devices in combination with apermanent magnet, and an impedance commutator utilizing a saturablereactor (U.S. Pat. No. 2,797,375) or the mutual coupling of two coils(U.S. Pats. Nos. 1,971,188 and 3,091,728) all operating on relativelyhigh frequency alternating current.

Many deficiencies of mechanically commutated motors, such as relativelyshort life due to the wearing of brushes and the commutator material,generation of electric, electromagnetic and sonic noise due to thesparking and chattering between the brush and the commutator, and energyloss due to the friction between the brushes and the commutatormaterial, are overcome. byusing any of the methods described above.

However, there do exist other difficulties in such brushless motors(hereinafter called electronically commutated motors). -Anelectronically commutated motor employing photosensitive elements doesnot have a long life or a high efficiency because the light sourceusually has a relatively short life and poor efficiency of conversion ofelectronic energy to light energy. The most prominent feature of theoptical system which utilizes photosensitive elements is that it is easyto give the commutation signal an on-off characteristic, or in otherwords, to provide a descrete signal level for on-off operation.

A discontinuous commutation signal is necessary for high efficiency. Themagnetic system and the impedance system are preferable to the opticalsystem fromthe stand point of their length of useful life, although thecommutation signal obtained by those systems is not a discontinuous one.

g The impedance system, which is very inexpensive as compared to theother systems, produces 'a commutation signal .which has very poordiscontinuity, especially where the system has a simple construction.

In addition to discontinuity, two important features of the commutationsignal are as follows:

i. No dead zone should exist when changing from one phase signal toanother phase signal. This will prevent incorrect starting.

ii. No overlap of one phase with another phase should exist. This willprevent small torque ripple and premote high efficiency.

These two conditions are very difficult to satisfy simultaneously withthe feature of good discontinuity in a commutation signal produced in anoptical system, and with the feature of poor discontinuity in acommutation signal produced in a magnetic system or an impedance system.

It is an object of the present invention to provide an improvedelectronically commutated motor.

his a further object of the present invention to provide anelectronically commutated motor having little torque ripple.

It is a still further object of the present invention to provide anelectronically commutated motor which does not start improperly.

It is a still further object of the present invention to provide anelectronically commutated motor which does not require too muchprecision in the manufacturing process.

It is a still further object of the present invention to provide aprinciple for the construction of a perfect commutation signal which canbe utilized in any electronic commutating system.

The electronically commutated motor, according to the present invention,has a commutation signal with the above mentioned important featureswithout the disadvantages inherent in the use of the optical system, themagnetic system or the impedance system.

It is a still further object of the present invention to provide amethod for controlling torque generated by a motor by a relatively smallpower dissipating variable resistor, without losing any of theabove-mentioned features.

It is a still further object of the present invention to provide amethod for controlling the torque of a motor by a weak electric signal,without losing any of the above-mentioned features.

It is still a further object of the present invention to provide amethod for controlling the torque of a motor driven from constant supplyvoltage, without losing any of the above-mentioned features.

It is a still further object of the present invention to provide aconstant torque motor.

It is a still further object of the present invention to provide a motorwhich can be applied to a feedback control system without using anyadditional power amplifier.

It is a still further object of the present invention to provide a speedcontrolled motor.

Briefly described, the motor according to the present invention utilizespolyphase rectification by a base-emitter circuit of a transistor, andthe common emitter impedance of the transistor is varied so as tocontrol the generating torque of the motor.

Other features and advantages of the present invention will,

of course, become apparent and immediately suggest them-.

selves to those skilled in the art to which the invention is directedfrom a reading of the following specification in connection with theaccompanying drawing in which:

FIG. 1 is a schematic circuit diagram of an electric motor apparatusaccording to the present invention;

FIG- 2 is a schematic diagram of another electric motor apparatusaccording to the present invention;

FIGS. 3a-3h are time diagrams for explaining the operation of the motorcircuits of FIGS. 1 and 2;

FIG. 4 is a graph illustrating curves of speed vs. torquecharacteristics of a DC motor having a circuit according to the presentinvention;

FIG. 5 is a graph illustrating curves of the speed vs. torquecharacteristics of a DC motor having a circuit according to the presentinvention; and

FIGS. 6a-6d are time diagrams for explaining the operation of the motorcircuit of FIG. 2.

In the embodiment shown in FIG. 1, a stator 1 has three stator windings2, 3 and 4 wound thereon.

A rotor 5 is rotatably positioned within stator l. A position detectingmeans 6 is provided which comprises a position detecting rotor 7 on theshaft of rotor 5, a primary winding 8, and three secondary windings 9,l0 and 11 equidistantly spaced around the detecting means 6. The primarywinding 8 is arranged in the center of the position detecting means 6and always faces the position detecting rotor 7. Each of the secondarywindings 9, l0 and 11 faces the position detecting rotor after it hasmoved 120 from the preceding winding.

A transistor 20, resistors 21, 22 and 23, capacitors 24 and 25, and anoscillator coil 26 are connected in an oscillator circuit whichgenerates a comparatively high frequency Hz-IOO KHz) AC signal.

The output signal of the oscillator circuit is fed to said primarywinding 8 through a capacitor 27.

Diodes l2, l3 and 14 are connected forwardly, with respect to currentflow from the secondary windings, between the one ends of the respectivesecondary windings and the respective bases of transistors 28, 29 and30. Each of the other ends of the secondary windings 9, l0 and 11 areconnected to one another and connected to a point at which a resistor 18and a Zener diode 19 are connected to each other, said point hereinafterbeing designated as pedestal point 55. Resistor l8 and Zener diode 19are connected in series across power supply lines from terminals 36 and37.

Capacitors 15, 16 and 17 are connected between the bases of thetransistors 28, 29 and 30 and the other ends of said secondary windings,respectively. p

The emitters of the transistors 28, 29 and 30 are connected to oneanother. A resistor 34 and a capacitor 35 are connected in parallelbetween a power supplying terminal 37 and said commonly connectedemitters of transistors 28, 29 and 30.

The bases of transistors 31, 32 and 33 are connected to the collectorsof the transistors 28, 29 and 30, respectively.

The emitters of the transistors 31, 32 and 33 are each connected to theother power supplying terminal 36.

The collectors of the transistors 31, 32 and 33 are connected to one endof said stator windings 2, 3 and 4, respectively and each of the otherends of said stator windings 2, 3 and 4 is connected to the powersupplying terminal 37.

Transistors 28, 29 and 30 have a polarity opposite to that oftransistors 31, 32 and 33; i.e., if transistors 31, 32 and 33 are PNPtype transistors, transistors 28, 29 and 30 are NPN type transistors andvice versa. Said oscillator circuit is energized by the current fed fromthe power supplying terminals 36 and 37.

In operation, the output signal of said oscillator circuit is fedthrough the capacitor 27 to the primary winding 8. A flux induced inposition detecting rotor 7 by the primary winding 8 induces a voltage inthe secondary windings 9, l and 11 in sequence during rotation of therotor 7.

Each of the voltages appearing at the ends of the respective secondarywindings has a frequency which is the same as the frequency of theoutput signal of the oscillator circuit, and has an amplitude varyingaccording to the rotational angle of position detecting rotor 7. It willbe understood that the position detecting rotor 7 modulates theamplitude of said voltage. The modulated signal is shown in FIGS. 3a-3c.ln FIGS. 3a, 3b and 3c, curves 45, 46 and 47 show the envelopes of theoutput signal of the respective secondary windings. These curves showthat said output signals are not greatly modulated, and the envelopesform a three phase curve family.

Diodes 12. 13 and 14 rectify said output signals from the secondarywindings 9. 10 and 11, and capacitors 15, 16 and 17 filter out thecarrier frequency, i.e. the frequency generated by the oscillatorcircuit. The voltages appearing between the pedestal point 55 and theoutput side (cathode in this case) of the respective diodes, aredesignated 2,, e and e and are shown in FIG. 3d as curves 48, 49 and 50.The voltage appearing between the pedestal point 55 and the powersupplying terminal 37 is indicated as being e, in FIG. 1.

The resistor 18 is given a resistance such that the current flowingthrough it is comparatively large compared to the base current oftransistors 28, 29 and 30.

The voltage of the Zener diode 19 is chosen so that the voltageappearing between the emitters of transistors 28, 29 and 30 and powersupplying terminal 37 (shown as e in FIG. 1) is large compared to thebase to emitter forward voltages of transistors 28, 29 and 30. Thediodes 12, 13 and 14 have an output impedance which is low compared tothe base circuit impedance of said transistors 28, 29 and 30.

The output signal of the secondary windings 9, and 11 are determined bythe value of the peak to valley voltage difference of the voltages e eand 2 When said peak to valley voltage difference is from about 0.5 toseveral volts, the transistors 28, 29 and 30 and resistor 34 act as atriple differential switching circuit.

The voltage e appearing between the common emitter circuit of saidtransistors 28, 29 and 30 and the power supplying terminal 37,corresponds to the greatest voltage of the combined voltages e e e e,and e;, e.,. For example, the position of the position detecting rotor 7shown in FIG. 1 is such that e e, is the greatest of the three combinedvoltages.

The transistor to which the highest base emitter voltage is applied, inthis instance transistor 28, feeds its emitter current to the resistor34 and the voltage e, is nearly equal to e, e. 0.6 volts (when thetransistors 28, 29 and 30 are silicon transistors). This state isdesignated as the ON state of the transistor 28. On the other hand, thebase-emitter voltages of the transistors 29 and 30 are very low and thebase current and the collector current can not flow. This state isdesignated as the OFF state of transistors 29 and 30.

The collector current of transistor 28, which is in the ON state, issupplied to the base of transistor 31, which then is turned to the ONstate to supply collector current to the stator winding 2.

The current flowing through the stator winding 2 generates a torque incooperation with permanent magnetized rotor 5. The rotation of the rotor5, and consequently the rotation of the position detecting rotor 7,varies the voltages e,, 2 and 2 If it is assumed that the torquegenerated by the current flowing through the stator windings 2 and therotor 5 has a clockwise direction, then when the rotor 5 rotates about60 from the position shown in FIG. 1, the output signals of thesecondary windings 9 and 10 have equal amplitudes and the voltages e ande become equal in value and the emitter current of transistor 28 isdecreased and the emitter current of transistor 29 is increased.

At this point the two transistors 28 and 39 are in the same state, andboth of them feed their emitter current to the resistor 34. With furtherrotation of rotor 5, the emitter current .of transistor 28 decreasesfurther and the emitter current of transistor 29 increases further. Thesum of the emitter currents of transistors 28 and 29 is determined bythe emitter voltage e and the resistance value of resistor 34. As thevoltage e follows to the base potential of transistors 28 and 29, itremains almost constant, as shown in FIG. 3e as curve 51. When theemitter current of each of transistors 28 and 29 has the same value,which is nearly equal to one half of the emitter current of a singletransistor when it is in the ON state, this state is called thetransitional state of transistors 28 and 29.

In the vicinity of the transitional state, the two transistors act as adifferential amplifier. But the maximum voltage difference of the twosecondary windings 9 and 10, i.e., the maximum difference of the inputsignal to the differential amplifying transistors 28 and 29, ispredetermined so as to be sufficiently large to overcome thedifferential operation and to drive one of the two transistors into theON state and the other into the OFF state. Thus the two transistorsoperate as a differential amplifier only for a very small rotationalangle of the rotor 7. Therefore, transistors 28 and 29 switch from an ONto an OFF state and vice versa almost instantaneously.

The change of the states of the transistors 29 and 30 and 30 and 28follow the same pattern as that for the transistors 28 and 29. The ONstate of each of the transistors 28, 29 and 30 continues for about l20of the rotation of the rotor 5. Therefore the rotor 5 generates torquein one direction all during its rotation. The collector currents of thetransistors 28, 29 and 30 are shown in FIGS. 3f-3h as curves 52, 53 and54. Capacitor 35 eliminates undesirable parasitic oscillation.

The switching system described herein is the fundamental principle ofthis invention. The position detecting device illustrated and describedis one which employs an impedance-type commutator utilizing mutualcoupling of two coils. But the switching system is applicable to almostall position detecting devices. A gradually varying commutating signalis converted to a commutating current having no overlap and no deadzones by a polyphase rectification gate circuit. As the motor embodyingthe present invention has no dead zones and no overalp, even if thetemperature varies, and even if the supply voltage to the power supplyterminal varies, perfect starting of rotation is guaranteed and verysmooth torque is obtained.

The smoothness of the torque causes good transformation of current totorque and high efficiency is obtained.

There are many cases in which it is necessary to control the torquegenerated by the rotor. Controlling of the torque generated by a DCmotor can be carried out by varying the supply voltage. However, it isnot economical to provide a voltage controlling device between a powersupply and the motor.

In the motor described hereinbefore, the generating torque is inproportion to the current flowing through the stator windings 2, 3 and4. These currents are controlled by the resistor 34, since the currentflowing through the stator windings 2, 3 and 4 to control the basecurrents of the transistors 31, 32 and 33 respectively, which are almostthe same as the current flowing through the resistor 34. As the pedestalvoltage e is large compared to the forward direction base emittervoltage of the transistors (about 0.6 V for silicon transistors), thetransistors 28, 29 and 30 operate as almost a constant current circuitwhen they are in an ON state, the current of which is controlled by theresistor 34.

Therefore, the current flowing through the stator windings is almostconstant during the time current flows. Constant current in the statorwindings produces a constant torque. Therefore an almost constant torquemotor can be obtained as shown in FIG. 4. When a sufficiently large basecurrent is supplied to the transistors 31, 32 and 33these transistorsare completely saturated even in the locked state, whereby theconventional characteristic of a shunt motor is obtained, which isillustrated by curve 59in FIG. 4. For lesser base currents, as governedby and almost equal to the current of resistor 34, the maximum value ofthe torque decreases, as indicated by curve 600 in FIG. 4. The less thecurrent flowing through the resistor 34, which is determined by thevoltages e e e;, and e indicated in FIG. I and the resistance value ofresistor 34. These voltages are almost independent of the power supplyvoltage.

In FIG. 1 the resistor 34 is designated as a variable resistor. Thecurrent flowing through the resistor 34 is 1/3 of the currents flowingthrough the stator windings 2, 3 and 4 where B is the currentamplification factor of the transistors 31, 32 and 33. Therefore thepower dissipation in the resistor 34 is very small compared to the inputpower to the stator windings, and a conventional variable resistor canbe used as the resistor 34.

,Any other variable resistance means, such as a CdS photoresistive cellfor example, and any current controlling device can be substituted forthe resistor 34.

A most conventional and most effective device is a transistor. When theresistor 34'is replaced by a transistor, the generated torque iscontrolled by the electric current to the base of the transistor. Thisindicates that the motor is able to be used in an electric servomechanism without using any power amplifier.

The most prominent feature of the motor according to the invention isthat the current flows continuously in successive stator windings andthe generated torque is very smooth even when the generated torque issmall compared to the full starting torque.

FIG. 2 shows an embodiment of a speed controlled motor which is anexample of the application of controlling the generated torque by meansof an electric signal.

The collector to emitter path of a transistor 38 is connected in seriesto the resistor 34. The one electrodes of diodes 39, 40 and 41 arerespectively connected to the one end of the stator windings 2, 3 and 4to which the collectors of the transistors 31, 32 and 33 arerespectively connected, in a blocking direction with respect to thecollector currents of the transistors 31, 32 and 33. The otherelectrodes of the diodes 39, 40 and 41 are connected together at a point58. A resistor 42 is connected between the base of the transistor 38 andthe point 58.

A capacitor 44 is connected between the point 58 and the power supplyterminal 37. A resistor 57 and a Zener diode 56 are connected in seriesand connected across the power supply terminals 36 and 37. A resistor 43is connected between the base of the transistor 38 and the junctionpoint of resistor 57 and the Zener diode 56.

In operation, the emitter currents of the transistors 28, 29 and 30 arecontrolled by the base current of the transistor 38. The collectorvoltage wave forms of the transistors 31, 32 and 33 are shown in FIGS.6a-6c. Diodes 39, 40 and 41 rectify the reverse voltage and a DC voltagecontaining a ripple voltage is obtained at the point 58, and is shown inFIG. 6d. This voltage is smoothed by the capacitor 44, and there isobtained a DC voltage, which is in proportion to the rotational speed ofthe rotor 5. The zener diode 56 provides a reference voltage at thejunction point of the resistor 57 and the Zener diode 56. The basevoltage of the transistor 38 varies according to the ratio of theresistance values of resistors 42 and 43 and the ratio of the voltagesof the speed controlling voltage appearing at the point 58 and thereference voltage of Zener diode 56. The base current of the transistor38 varies greatly when its base voltage is about 0.5-0.7V.

The resistors 43 and 42 are given values so that the ratio of theirresistances is nearly equal to the ratio of the reference voltage andthe voltage of the point 58 corresponding to the predetermined speed.

Then, if the speedof the rotor 5 exceeds the predetermined speed, thebase voltage of the transistor 38 decreases, causing a decrease in thebase current of the transistors 31, 32 and 33, and the generated torqueis decreased. The decrease of the generated torque results in a decreaseof the speed of the rotor 5. When the speed of rotor 5 decreases belowthe predetermined speed, the base current of the transistor 38 increasescausing an increase of the base currents of the transistors 31, 32 and33, and the generated torque is increased. The increase of the generatedtorque increases the speed of the rotor 5. Therefore, the speed of therotor 5 is regulated so as to have an almost constant speed of apredetermined value.

In FIG. 5, curves 61 and 62 illustrate the characteristics of the casewhen the collector to emitter path of the transistor 38 is shorted by aconductor. Curves 63, 64, 65 and 66 illustrate the characteristics ofthe case when the transistor 38 operates and the speed of the rotor isregulated at some predetermined value. To give a predetermined speed tothe rotor it is only necessary to provide the proper resistor 42 orresistor 43 or a Zener diode 56 of the proper voltage.

The speed of the rotor is thus almost independent of any variation inthe power supply voltage.

We claim:

1. An electric motor apparatus comprising stator windings mounted forgenerating magnetic fields in selected areas; a permanently magnetizedrotor movably mounted adjacent said stator windings for rotation withinsaid magnetic fields; a position detecting means coupled to said rotorfor detecting the relative rotational position of said stator windingsand said rotor, said position detecting means including voltagegenerating means for generating a family of voltages having valuesvarying according to said rotational position; a current control circuitcomprising a control terminal and a current path; a plurality oftransistors, the emitter of each of said transistors being coupled tothe current path of said current control circuit; means for generating apedestal voltage to which said voltage generating means is coupled foradding said pedestal voltage to each of said family of voltages, theoutput of said pedestal voltage generating means being coupled with thebases of said transistors and the current path of said current controlcircuit for supplying the voltage generated by said position detectingmeans across the bases of said transistors and the current path of saidcurrent control circuit; means coupling the collectors of saidtransistors to the respective stator windings; means for detecting therotational speed of said rotor generating a signal correspondingto thespeed; means for generating a reference signal to provide a signalcorresponding to the predetermined speed; and means for comparing saidsignal corresponding to the speed and said reference signal andobtaining a signal to control said control circuit and coupled to saidterminal of said current control circuit.

2. An electric motor apparatus comprising:

three sets of stator windings mounted for generating magnetic fields inselected areas;

a permanently magnetized rotor movably mounted adjacent said statorwindings for rotation within said magnetic fields;

a position detecting means having a position detecting rotor coupledwith said permanently magnetized rotor and rotatable therewith, andposition detecting stator means positioned around said positiondetecting rotor, said position detecting stator means generating threesets of 10 gradually varying position signals in cooperation with theposition detecting rotor during rotation thereof, said position signalindicating the relation of the relative rotational position of saidstator windings and said'rotor and being gradually varying signals whichhave a poor on-off ratio;

a pedestal voltage generating means to which said position detectingmeans is coupled for providing a direct current bias voltage to theposition signals;

an impedance means having two terminals and the impedance of which isvariable in response to an external command; three transistors, theemitter of each of said transistors being coupled to one terminal ofsaid impedance means, said position detecting means being coupled to thebases of said transistors and to the pedestal voltage generating meansfor supplying the signals generated by said position detecting means andthe pedestal voltage means across the bases of said transistors and theother terminal of said impedance means; and

means coupling the collectors of said transistors to the respectivestator windings.

3. An electric motor apparatus as claimed in claim 2, in which saidimpedance means is a variable resistor.

4. An electric motor apparatus as claimed in claim 2. in which saidimpedance means is a variable impedance means responsive to a physicalvalue.

5. An electric motor apparatus as claimed in claim 2, in which saidimpedance means is a resistor and a transistor.

1. An electric motor apparatus comprising stator windings mounted forgenerating magnetic fields in selected areas; a permanently magnetizedrotor movably mounted adjacent said stator windings for rotation withinsaid magnetic fields; a position detecting means coupled to said rotorfor detecting the relative rotational position of said stator windingsand said rotor, said position detecting means including voltagegenerating means for generating a family of voltages having valuesvarying according to said rotational position; a current control circuitcomprising a control terminal and a current path; a plurality oftransistors, the emitter of each of said transistors being coupled tothe current path of said current control circuit; means for generating apedestal voltage to which said voltage generating means is coupled foradding said pedestal voltage to each of said family of voltages, theoutput of said pedestal voltage generating means being coupled with thebases of said transistors and the current path of said current controlcircuit for supplying the voltage generated by said position detectingmeans across the bases of said transistors and the current path of saidcurrent control circuit; means coupling the collectors of saidtransistors to the respective stator windings; means for detecting therotational speed of said rotor generating a signal corresponding to thespeed; means for generating a reference signal to provide a signalcorresponding to the predetermined speed; and means for comparing saidsignal corresponding to the speed and said reference signal andobtaining a signal to control said control circuit and coupled to saidterminal of said current control circuit.
 2. An electric motor apparatuscomprising: three sets of stator windings mounted for generatingmagnetic fields in selected areas; a permanently magnetized rotormovably mounted adjacent said stator windings for rotation within saidmagnetic fields; a position detecting means having a position detectingrotor coupled with said permanently magnetized rotor and rotatabletherewith, and position detecting stator means positioned around saidposition detecting rotor, said position detecting stator meansgenerating three sets of gradually varying position signals incooperation with the position detecting rotor during rotation thereof,said position signal indicating the relation of the relative rotationalposition of said stator windings and said rotor and being graduallyvarying signals which have a poor on-off ratio; a pedestal voltagegenerating means to which said position detecting means is coupled forproviding a direct current bias voltage to the position signals; animpedance means having two terminals and the impedance of which isvariable in response to an external command; three transistors, theemitter of each of said transistors being coupled to one terminal ofsaid impedance means, said position detecting Means being coupled to thebases of said transistors and to the pedestal voltage generating meansfor supplying the signals generated by said position detecting means andthe pedestal voltage means across the bases of said transistors and theother terminal of said impedance means; and means coupling thecollectors of said transistors to the respective stator windings.
 3. Anelectric motor apparatus as claimed in claim 2, in which said impedancemeans is a variable resistor.
 4. An electric motor apparatus as claimedin claim 2, in which said impedance means is a variable impedance meansresponsive to a physical value.
 5. An electric motor apparatus asclaimed in claim 2, in which said impedance means is a resistor and atransistor.